The heart is a muscle that continuously pumps blood, transporting life sustaining oxygen and nutrients to the major organs and tissues of the body. As such, the heart needs a large supply of oxygen and nutrients. The heart receives its blood from three major coronary arteries. If any one of these arteries becomes narrowed or blocked, blood flow and oxygen to the heart is restricted, the performance of your blood pump is impaired, and permanent damage to the heart can occur. An impaired or damaged heart can significantly impact quality of life and even result in death.
Coronary artery disease (CAD) is the primary cause of narrowing and blockage of the arteries to or in the heart. It is the leading cause of death in the United States. Although the rate of death due to CAD has declined steadily over the last few decades, the overall incidence of CAD and the impact it will have on the population and the cost of health care is expected to grow dramatically over the next 20-30 years due to the aging population. CAD already costs the U.S. an estimated $100 billion annually in medical care and lost productivity. Therefore, the tests for CAD that can improve the accuracy of cardiac evaluations, particularly in early stage coronary heart disease/CAD, may be beneficial. Tests that may allow physicians to make explicit diagnoses and treatment plans, that may reduce unnecessary tests, surgeries and hospital stays, may reduce anxiety, discomfort and risk to the patient and/or may allow for the assessment of therapeutic efficacy may be particularly beneficial.
The ability to mitigate and circumvent the effects of CAD has improved over the years as a result of the prior development of increasingly sophisticated cardiac tests and procedures. The “gold standard” of cardiac tests is generally recognized as cardiac catheterization and angiography. This test is most commonly used in determining the location and the severity of the effects of CAD. However, a cardiac catheterization procedure is an expensive procedure that typically requires hospitalization because it is invasive and has potential for complications. As a result, cardiac catheterization procedures are generally only used when a patient has undergone other non-invasive tests such as cardiac stress tests with significant abnormal results. More than one million cardiac catheterization procedures are performed each year at a cost estimated to be between about $3-$5 billion.
Cardiac stress tests may be particularly important in evaluating the heart and its coronary arteries because often the presence of CAD and its effects are missed when tests are performed on a patient at rest. It is only when the heart is stressed by either exercise on a treadmill or exercise bike, or where the maximal exercise is simulated by the introduction of special chemicals or drugs (drug-induced stress) to the heart of patients who for various reasons are unable to perform actual physical exercise, that the symptoms of CAD can be reliably detected over much of the population. Currently, the most commonly used stress tests include the exercise (treadmill) stress test, the stress echocardiogram (ultrasound), and the nuclear perfusion stress test.
These three tests have varying levels of complexity, accuracy, availability, and cost. The ECG exercise stress test is the most widely available and used stress test and the least costly to administer. It relies on detection of changes in ST segments on a 12-lead echocardiogram; it is variable in its accuracy, producing a significant number of false positive and negative results. Consequently, patients often undergo supplemental imaging during their stress test with echocardiography or radionuclide scintigraphy. The sophistication of both echocardiography and radionuclide scintigraphy stress tests typically require that they be administered and evaluated by a specialist. Typically, a trained ultrasound technician or a cardiologist administers a stress echocardiogram, and a technician certified in the handling of radioisotopes along with a cardiologist or radiologist administers a nuclear perfusion stress test. Results in both testing protocols generally need to be evaluated by a cardiologist in order to make an accurate diagnosis. The test selected for administration is typically determined by the cardiologist based on the findings of the physical and medical history of the patient and the cardiologist's clinical judgment. Each test has a degree of inaccuracy. The echocardiography has an almost 10-15% failure rate where a diagnosis cannot be made. Obstacles that may prevent the capture of good images for diagnosis may cause this failure rate. Scar tissue from chest surgeries, excessive patient body fat and lung disorders that capture excessive oxygen in the lungs are examples of such obstacles. Stress echocardiography tests, for varying reasons, also produce a significant number of false negative results. The nuclear perfusion stress tests are highly sensitive and are prone to produce an equally significant number of false positive results.
With over 1.3 million stress echocardiography tests performed each year with a 10-15% failure rate, there are potentially over $100 million unnecessary or clinically unreliable stress echocardiography tests done each year. This stress echo failure rate results in an increase in the number of nuclear perfusion stress tests, which are known to have a high incidence of false positive results. A false positive cardiac stress test may increase the number of expensive, unnecessary cardiac catheterization procedures and their associated required hospital stay and added anxiety, discomfort and medical risk for the patient. The significant number of false negatives from stress echocardiography tests may increase the risk of heart attacks and sudden death from undetected advanced CAD and/or successful earlier intervention in early stages of CAD.
Accordingly, a need exists for reduced cost, increased accuracy and/or increased availability stress testing for cardiac diagnostics.